1. Field of the Invention
The present invention relates to an integrated optoelectronic thin-film sensor for a measuring system, and a method for producing it.
2. Description of the Related Art
A measuring system is known from GB 1,504,691 and corresponding DE 25 11 350 A1, wherein the displacement of a first component in relation to a second component is a determined. Two gratings are provided for this, which are at a constant distance from each other and each one of which is respectively fastened on one component. If the second grating is illuminated with divergent light, the first grating generates a periodic image of the second grating, which image moves if there is a relative movement between the two components. Photodetectors, which have a periodic structure and are fixedly connected with the second component, are furthermore provided. In this case the first one is a reflecting grating, and the second grating and the photodetectors are essentially located in one plane. The light source and the second grating can also be replaced by a structured light source, which generates the same image as a conventional light source and a grating. The structure of the photodetectors interacts with the image in such a way that a periodic change of the output signal from the photodetector occurs if a relative movement exists between the first and second components.
It is disadvantageous here that the individual components are discretely and separately provided. Because of this, a relatively large installation space is required for the entire arrangement.
It is known from DE 197 01 941 A1 that a scanning grating is arranged on the side of a transparent substrate facing a scale. The scanning grating is illuminated by a light source in such a way that an image of the grating is projected onto the scale. A second grating is located on the scale, which reflects the image onto a structured photodetector. In this case the transparent substrate for the first grating is connected with the semiconductor material from which the structured photodetector is made in such a way that the scanning grating and the photodetector are exclusively offset from each other in the measuring direction, but are at the same distance from the scale. In a second embodiment of DE 197 01 941 A1, the scanning grating is arranged on the side of the transparent substrate which is facing away from the scale. An optical chip, which contains the photodetector, is arranged on the same side of the same transparent substrate as the scanning grating. By means of this arrangement it is also achieved that the scanning grating and the structured photodetector have approximately the same distance from the scale.
In connection with the first embodiment there is the disadvantage that the transparent substrate to which the scanning grating is applied must be connected with the semiconductor material of which the structured photodetector is made. This connection must be performed very accurately, so that the structure of the photodetector is aligned parallel with the grating, and that the structure and the grating have the same distance from the scale. This exact connection between the substrate and the semiconductor material is therefore very difficult to provide. Furthermore, the second embodiment has the disadvantage that an optical chip must be fastened on the transparent substrate. Because of the attachment by means of a chip-on-glass technique, a space unavoidably exists between the optical chip and the substrate, because of which the distances between the transmitting grating and the scale, as well as between the photodetector and the scale, differ considerably, which results in a clear decrease of the optical properties of the arrangement.
It is known from DE 40 91 517 T1 to make a sensor for a measuring system out of a single block of semiconductor material. Here, photo elements designed as grating lines are provided on the surface of a flat-designed light-emitting diode, through which the light-emitting diode cannot radiate. A structured photodetector is created in this way, above or below of which a structured light source is arranged.
This sensor has the disadvantage that the photodetector structure and the structured light source unavoidably cannot have the same distance from a scale, since the light-emitting diode and the photodetector are placed on top of each other. This difference in the distance from the scale also clearly decreases the optical properties of the sensor.
It is known from EP 543 513 A1 to provide a structured photodetector, as well as a structured light source in the form of at least one light-emitting diode of a sensor on a common semiconductor layer of III/V semiconductor material, for example gallium arsenide GaAs. It is possible by means of providing the structured light source and the structured photodetector on a common semiconductor material to meet the requirement of providing the transmitting and receiving structures possibly in one plane very satisfactorily. Moreover, single field scanning takes place, wherein the photo elements are offset by 90xc2x0+k*360xc2x0, where k is a whole number. Thus, several photo elements are arranged offset in the measuring direction by ninety degrees of angle plus whole-number multiples of three hundred sixty degrees of angle. Because of this, scanning becomes particularly insensitive to interferences.
The disadvantage here is that it is not described how the production of the structured photodetector and the structured light source on a common semiconductor material of GaAs takes place. If technique known from the prior art are used for producing the semiconductor, this manufacturing process is very elaborate and therefore expensive.
An optical sensor for a measuring system, having a light-receiving component and at least one optical component which acts on the light beam transmitted by the component emitting the light before it arrives at the component receiving the light, is known from EP 720 005 A2. This sensor has a spacer element, which defines the distance between the component emitting, or respectively receiving, light, and the optical components. In this case the spacer element is embodied in such a way that it is connected with another component. It is achieved by means of this that the optical sensor transmits and receives optical signals on one of its sides, because of which optical elements are arranged on this side, and has conductors for electrical signals on the other of its sides.
In connection with this it is disadvantageous that the component receiving light, the component transmitting light, the at least one optical component and the spacer element are all separate components, which must be produced and assembled separately. This is very expensive in view of the required accuracy of optical sensors in measuring systems. Moreover, the optical sensor is rather voluminous, since the individual components also must be separately handled.
An electronic hybrid component is known from DE 197 20 300 A1, wherein an implanted chip is placed in a chip-on-chip arrangement on a support substrate. To this end, the support substrate has at least one cavity, in which an electrical insulation layer with a metal layer on top of it is located. The chip implanted in the cavity is connected to the metal layer, because of which the latter is used as an electrical conductor. If the implanted chip is a light-emitting diode, the metallized layer can also be used for reflecting the radiation from the latter at the walls of the cavity.
This arrangement has the disadvantage that the direction of radiation from the light-emitting diode, as well as its electrical contacts, are arranged on one side of the semiconductor layer, or are emitted from this one side.
A radiation-sensitive detector element with an active area is known from DE 196 18 593 A1, wherein the active area is formed between two adjoining layer areas of a layer arrangement of different charge substrates, in which a conversion of incident electromagnetic radiation into electrical signals takes place. Here, the position of the active area in relation to the two bordering surfaces is selected, taking into account the penetration depth of the radiation, in such a way that at least two contact elements for connecting the detector element with an evaluation circuit can be mounted on a surface located opposite the radiation-sensitive surface on which the incident radiation strikes. The following process steps are employed in the course of the production method for such a detector element. An etch stop layer is created in a specifically doped semiconductor layer closely below a limiting first surface. This is followed by a spatially selective removal by etching of the substrate present underneath the etch stop layer until the etch stop layer forms a limiting second surface. Thereafter, a spatially limited layer area is created above the etch stop layer, which is doped differently from the semiconductor layer, and the detector element is connected to a side opposite the second surface by means of at least two contact elements.
The disadvantage here is that only a photodiode is disclosed here, but not a complete optoelectronic sensor.
A scanning head for a scale with a graduation is known from DE 198 59 670.7, wherein the scanning head essentially includes a single semiconductor layer having several structured photodetectors on the side facing the scale, which furthermore has a blind bore which is bordered by transmitting grating on the side facing the scale. In this case the transmitting grating is formed in a layer placed on the semiconductor layer, or by a special embodiment of the process for drilling the blind bore. A light source is moreover arranged in the blind bore, which radiates in the direction toward the transmitting grating.
It is disadvantageous here that a relatively large distance exists between the transmitting grating and the light source. It is also disadvantageous that the photodetectors are connected to the side facing the scale.
An integrated optoelectronic sensor for scanning a graduation is known from DE 198 59 669.3. The sensor includes a single semiconductor layer, which has several detectors on the side facing away from the graduation. The semiconductor material has been at least partially removed in the area of the photodetectors, so that light can enter into the light-detecting area of the photodetectors from the side of the sensor facing the graduation. The sensor has a light source on the side facing away from the graduation, in whose area the entire thickness of the opaque components is disrupted, so that the light source radiates through the sensor. On the side facing the graduation, the semiconductor layer is connected with a transparent substrate member having at least one further graduation, by which the radiation from the light source is optically affected.
It is disadvantageous here that the at least one graduation applied to the substrate member in the area of the photodetectors is relatively far away from the photodetectors, and in the area of the light source relatively far away from the light source. The optical properties are worsened by this. A further disadvantage lies in that during the production it is necessary to remove semiconductor material in the area of the photodetectors in a special method step.
It is therefore an object of the present invention to disclose an integrated optoelectronic sensor and a method for producing it, wherein the distance between the optical gratings and the photodiodes, as well as the light source, is as short as possible. In this case the production method for the integrated optoelectronic sensor should be as cost-effective as possible.
This object is attained by a measuring system that includes a scale and a transparent substrate located opposite the scale. The transparent substrate includes a graduation structure and a semiconductor layer arranged on a first side of the transparent substrate facing away from the scale, wherein a photodetector, a light source and an electronic circuit are integrated into the semiconductor layer.
The sensor of the present invention has the advantage that the semiconductor layer in which the photodetectors are formed is designed to be very thin. Because of this the distance between the gratings applied to the substrate and the photodetectors, as well as the between the light source and the photodetectors, is very short. The optical properties of the sensor are improved by this. A further advantage lies in that no special process steps are required for reducing the thickness of the semiconductor layer in the area of the photodetectors. The semiconductor layer is already so thin that the detecting areas of the photodetectors extend as far as the boundary with the substrate.
Further advantages, as well as details of the sensor in accordance with the present invention and of the method in accordance with the present invention ensue from the following description of an exemplary embodiment by means of the drawings. Shown are in: